The Giants spent all week leading into Sunday’s must-win game preparing for Philadelphia quarterback Michael Vick’s legs. Turned out it was the mid-game switch to backup Nick Foles’ arm that handed them a 36-21 loss and may have sunk their season.

Vick did run seven times for 79 yards, but the Giants held him to just 6-of-14 passes for 105 yards and a 69.0 QB rating. But once Vick pulled up lame with a tight left hamstring, Foles came on in relief and torched the Giants, going 16-of-25 for 197 yards, two touchdowns, no interceptions and a 114.9 rating.

“I think our coaches and scout team did a great job in simulating [Vick] in practice and I definitely think that we were prepared for that,’’ said cornerback Prince Amukamara. “We were well-conditioned coming into this game, but when Mike Vick got out, their whole game plan changed just because there aren’t two runners in the backfield. Foles does more passing.

“It’s a midstream adjust. All week we prepared just for them to run the read option but it was just different.’’

Vick pulled up lame after a 13-yard run in the second quarter, but started the final drive of the half before defensive end Jason Pierre-Paul fell on him in the end zone. He left and didn’t return, but Foles went 7-for-8 on that drive, leading the Eagles to a field goal as time expired and a 19-7 lead.

The Giants stuck with their man-to-man coverages, but never seemed to adjust well to Foles. He tossed a 25-yard touchdown to Brent Celek, and a 6-yard touchdown to DeSean Jackson, who beat Amukamara badly.

“Vick not being in the game changes our game plan a bit, because we know he was not as big a threat to run with Foles because he’s not so much a mobile quarterback,’’ said linebacker Spencer Paysinger. “He can get a couple yards he needs but that’s not the type of quarterback he is. So it changes up the game plan.

“He’s a professional quarterback. He got in there and got the right calls, got the right reads. You can’t take anything away from him: He was a starting quarterback for X amount of games for them last year, so just because Vick went out we’re not going say a scrub came in so we should to automatically win. He’s in the NFL for a reason and he showed time and time again last year that he can win games.’’